Dynamic chart title created in VBA does not auto-update

Hi -
I have used the following code to create a dynamic chart title that gets its value from cell Sheet1!$A$1.

activechart.title.caption = "=Sheet1!$A$1"

When the macro is run, the chart title initially displays what is in cell Sheet1!$A$1 correctly. However, I find that if the value in Sheet1!$A$1 changes, the chart title does not update.
If I manually link the chart title (select chart title and enter "=Sheet1!$A$1" into the formula bar) then the chart title updates appropriately when the cell value changes. When this is done using VBA, the chart title does not change when the linked cell value changes.

Thanks for your suggestions.